To evaluate the effects of coronary bypass surgery on disabling, chronic, stable angina, 100 patients were randomized to medical therapy (49) or surgery (51). New entries to the study were closed after 5 years in early 1977. All patients were in Functional Class III at the time of entry and initial clinical and angiographic data were similar in the medical and surgical groups. All patients undergo coronary arteriography, left ventriculography, myocardial metabolic study and graded exercise stress testing initially, 6 months after entry and after 5 years as well as regular clinical follow-up. Prospectively defined major cardiac events requiring termination from the study including death, myocardial infarction and unstable angina unresponsive to medical therapy and requiring surgery. There is no significant difference in terminating cardiac events after 4 years (death-5 medical vs. 4 surgical, infarction-11 vs. 13, unstable angina requiring surgery-9 vs. 4). Actuarial survival curves show no significant differences for death, infarction or for all terminating events. Surgical patients with three-vessel disease have had fewer major events (p less than 0.05) and less unstable angina requiring surgery (p less than 0.02). All unstable angina, including that requiring surgery and that responding to medical management is significantly more frequent in medicaly treated patients (16 vs. 7, p less than 0.05). Functional Classification improved more in surgical patients after 6 months (p less than 0.01) but is not significantly different at the latest follow-up averaging 4 years. However, surgical patients have achieved significantly higher exercise work loads (p less than 0.01) and exercise heart rates (p less than 0.05) during stress electrocardiograhy. Left ventricular ejection fraction and end-diastolic pressure are unchanged and not significantly different between the two groups in the 6 month and 5 year follow-up studies.